1. Field of the Invention
The present invention is directed to an ignition coil, and in particular to the ignition coil that is for use in an internal combustion engine.
2. Description of the Related Art
FIG. 3 is a cross-sectional view showing the conventional ignition coil of an internal combustion engine. In FIG. 3, a primary, bobbin 1 is constructed, in the form of a cylinder, of resin material such as PBT (polybutylene terephthalate). A primary coil 2 is formed by winding conductor wire around the circumference of the primary bobbin 1. A secondary bobbin 3 is constructed, in the form of a cylinder, of resin material such as PBT, and surrounds the primary bobbin 1 with which the secondary bobbin 3 is concentric. The outer circumference of the secondary bobbin 3 is formed of circular recessed portions and projected portions, with both portions alternate with each other in a comb-like fashion. A secondary coil 4 is formed by winding in the circular recessed portions around the secondary bobbin 3, conductor wire that is substantially large in the number of turns, compared to that the primary coil 2. The secondary coil 4 is wound in a block in each circular recessed portion around the circumference of the secondary bobbin 3, thus blocks arranged axially from the rightmost recessed portion to the leftmost recessed portion along the secondary bobbin 3 in FIG. 3.
One end of the primary coil 2 is terminated with a primary terminal 5, and the other end of the primary coil 2 is terminated with a primary terminal 6. The primary terminal 5 is connected to a power supply (not shown), and the primary terminal 6 is connected to a switching element (not shown). One end of the secondary coil 4 is terminated with a secondary terminal 7 where a high tension voltage is induced. The other secondary terminal (not shown), to which the other end of the primary coil 4 is connected is connected, to the secondary terminal 5 of the primary coil 2.
A casing 8 houses both the primary bobbin 1 around which the primary coil 2 is wound and the secondary bobbin 3 around which the secondary coil 4 is wound, wherein the secondary bobbin 3 is concentric with the primary bobbin 1. The casing 8, constructed of resin such as PBT, is provided with a support 8a for supporting the secondary terminal 7 on the left-hand side and a support 8b for supporting the primary terminals 5, 6 on the right-hand side. A core (iron core) 9 is made of an interior portion 9a that extends through the primary bobbin 1 and the casing 8, an exterior portion 9b that is external to the casing 8 and a ring connecting portion that connects the interior portion 9a and the exterior portion 9b. The core 9 magnetically couples the primary coil 2 with the secondary coil 4. The casing 8 is filled with insulating resin 10, such as epoxy resin, so that conductor components such as the primary coil 2 and the core 9 are insulated from high tension voltage components such as the secondary coil 4 and the secondary terminal 7.
Discussed next is the operation of the ignition coil. A current is conducted to the primary coil 2 via the primary terminal 5, causing magnetic flux in the core 9. When the current conducted through the primary coil 2 is switched on and off in accordance with the ignition timing of the internal combustion engine under the control of the switching element that is connected to the primary terminal 6, a high tension voltage develops, based electromagnetic induction, at the secondary terminal 7 of the secondary coil 4 according to the ratio of the number of turns of the primary coil 2 to the number of turns of the secondary coil 4. A discharge takes place at a spark plug connected to the secondary terminal 7, driving the internal combustion engine into motion.
The insulating resin 10 filled between conductor components such as the primary coil 2, the core 9 and high-tension voltage components such the secondary coil 4, the secondary terminal 7 serves as an insulator for insulating the conductor components from the high-tension components. The insulating resin 10 filled between the high-tension components such as the secondary coil 4, the secondary terminal 7 and the casing 8 serves as an insulator between conducting devices disposed in the vicinity of the ignition coil and the high-tension components.
In the conventional ignition coil described above, however, when a high-tension voltage develops at the secondary coil 4 and the secondary terminal 7, the insulating resin 10 suffers from localized discharge that originates in cavities that may exist in a small quantity in the insulating resin 10. When localized discharges are repeated, deterioration due to localized discharges advances from the secondary coil 4 or the second terminal 7 to the primary coil 2 and the core 9 inside the casing 8, subsequently leading to a dielectric breakdown between the high-tension components, such as the secondary coil 4 and the secondary terminal 7, and the low-tension components, such as the primary coil 2 and the core 9.
In the conventional ignition coil, the insulating resin 10 thus exhibits a relatively small resistance to localized discharge deterioration, thereby lowering reliability of the device against breakdown. The relatively small resistance of the insulating resin 10 against localized discharge deterioration means a larger separation requirement between the high-tension voltage components and other components. This presents difficulty in an effort to achieve a compact design.